[how to] Create an Ewallet ?

Hello world,
I'm looking to create an Ewallet (*multiple currencies)
And haven't found here a plugin that do such a thing, is this correct ?

I found this one @Woo :
But it's like adding a new product in the /shop/ and people can not indicate their own amount ... So I've found :
*which could be a solution

Before i go on with this, I would like to know if someone could have a better solution
Thanks for any tip you may have